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In computer programming, the verb glob or globbing is used to refer to an instance of pattern matching behavior. The noun "glob" is sometimes used to refer to a particular pattern, e.g. "use the glob *.log to match all those log files".
Technically, Unix globbing operates by parameter expansion – the glob pattern (say, *.log) is expanded and replaced by the list of all matches. For example, if a directory contains two files, a.log and b.log then the command cat *.log will be expanded by the shell to cat a.log b.log which is then evaluated (in this case, displaying the files). The order of arguments to a command often matters – for example, cat a.log b.log prints first a.log and then b.log, while cat b.log a.log prints first b.log and then a.log. Thus, while "filenames that match the pattern" is an (unordered) set, the actual expanded list of matching files is an ordered list, a sequence, and thus an order must be chosen, conventionally alphabetical order, however defined by the shell.
The term "glob" is also used to refer more generally to limited pattern-matching facilities of this kind, in other contexts:
- Larry Wall's book Programming Perl discusses glob in the context of the Perl language.
- Tcl contains both true regular expression matching facilities and a more limited kind of pattern matching often described as globbing.
- Python has a glob module in the standard library which performs wildcard pattern matching on filenames. Guido van Rossum, author of the Python programming language wrote and contributed a glob() routine to BSD Unix in 1986. There were previous implementations of glob, e.g., in the ex and ftp programs in previous releases of BSD.
- Perl has a Glob extension which mimics the BSD glob routine.
- Ruby has a glob method for the Dir class which performs wildcard pattern matching on filenames. Several libraries such as Rant and Rake provide a FileList class which has a glob method or use the method FileList. identically.
- PHP has a glob function.
Although there is no definite syntax for globs, common features include:
|Task||Example||Unix shells||Windows Powershell||SQL|
|Match one unknown character|
|Match any number of unknown characters|
|Match a character as part of a group of characters||N/A||N/A||N/A|
Globs do not include syntax for the Kleene star which allows multiple repetitions of the preceding part of the expression; thus they are not considered regular expressions, which can describe a larger set of regular languages over any given finite alphabet.
The command interpreters of the early versions of Unix (1st through 6th Editions, 1969-75) did not expand wildcard characters in file path arguments to a command; a separate program, /etc/glob, performed the expansion and supplied the expanded list of file paths to the command for execution.
- ↑ The "Advanced Bash-Scripting Guide, Chapter 19.2: Globbing" (Mendel Cooper, 2003) has a concise set of examples of filename globbing patterns.
- ↑ 'Globbing' library routine
- ↑ File::Glob - Perl extension for BSD glob routine - search.cpan.org
- ↑ PHP: glob - Manual
- ↑ First Edition Unix manual 'Miscellaneous' section (PDF)